This invention relates to a draft roll system for a flyer frame, spinning frame or the like in which the main draft is imparted to a sliver or a roving after imparting a preliminary draft by a plurality of draft rolls rotating at different speeds.
In a spinning machine in general and especially a spinning frame, as shown in FIGS. 1 and 2, a preliminary draft is imparted to a roving R by passing the roving through a nip between a back bottom roll 1 and a back top roll 1' kept in pressure contact with each other, and through a nip between a "mid bottom" roll 2 and a "mid top" roll 2 kept in pressure contact with each other and rotating at a faster speed than the set of back rolls 1, 1'. The main draft is then imparted to the roving by passing it through a nip between a front bottom roll 3 and a front top roll 3' also kept in pressure contact with each other and rotating at a faster speed than the set of "mid rolls" 2, 2'. Finally, the roving is drawn out from the nip between the front rolls 3, 3' and twisted into yarn, which is then taken up on a bobbin (not shown) fitted onto a spindle 7. The spindle 7 is rotated responsive to rotation of the drive shaft 4 which is connected to an electric drive motor (not shown) through a pulley 5 mounted on shaft 4 and a driving bolt 6.
Heretofore, the driving force for the bottom rolls 1, 2, 3 was derived from the drive shaft 4 through a gearing unit connected to one of the ends of each of the rolls as shown in FIG. 1, the other ends of the bottom rolls 1, 2, 3 being supported to rotate freely. However, in a spinning machine having a large number of spindles, the bottom rolls 1, 2, 3 tend to have increased length. Thus upon start-up the spinning machine, rotation of the bottom rolls 1, 2, 3 at their freely supported other end is delayed through torsion occurring at said other ends, that is, rotation of the bottom rolls at the one of their ends is not synchronized with rotation of the rolls at their outer ends. The spun yarn obtained by the draft roll system suffering from this phenomenon is not uniform in quality and problems may be caused, such as yarn breakage or unevenness.
Such torsion on the bottom rolls 1, 2, 3 is also caused by a larger rotational resistance offered by these rolls. The pair of aprons 8, 8' provided on the "mid-bottom" roll 2 and "mid-top" roll 2' for improving drafting conditions as shown in FIG. 2 tends to increase the rotational resistance of "mid-bottom" roll 2 (hereafter called apron bottom rolls). The result is that a larger load is placed on the apron bottom roll 2, thus frequently causing roll breakage.
In an attempt to remove such deficiency, it has also been known to provide a gearing similar to the gearing shown in FIG. 1 on the free ends of the bottom rolls 1, 2, 3 for reducing torsion on the bottom rolls 1, 2, 3. However, in such case, when the speed ratio between the respective bottom rolls is changed with changes in drafting conditions, it becomes necessary to change the speed change gears in both of the gearing units. Should an error be caused in the gear changing operation such that the rotational speed is not coincident at both ends of the bottom rolls, severe accidents may be caused such as breakage of the bottom rolls of destruction of the gearing units.
In the case where the lengths of the bottom rolls 1, 2, 3 are increased with an increase in the number of spindles in the spinning frame or unit, it may also be envisioned to use bottom rolls 1, 2, 3 of a larger diameter to increase their strengths for thereby preventing the torsion and the resulting risk of roll breakage. However, because of limitations upon the fiber length in the roving or silver, it is not possible to increase the distance between the respective bottom rolls 1, 2, 3, which are necessarily placed close to one another. Especially, a larger gage of spacing between the back bottom roll 1 and the apron bottom roll 2 results in the formation of stray fibers, thus again causing yarn unevenness. For these reasons, it is not possible to simply increase the diameter of the bottom rolls responsive to an increase in the number of spindles in the spinning unit.
It should also be noted that the apron 8 wound about the apron bottom roll 2 tends to become worn out or contaminated and hence needs to be replaced or cleaned frequently. The operation of mounting and demounting the apron 8 for such exchange or cleaning would be highly troublesome if the apron bottom roll 2 were to be elongated to cope with an increase in the number of the spindles in the spinning unit.
Hence, there exists a demand for a draft roll driving device, whereby torsion on the bottom rolls used in a spinning machine comprised of a number of spindles may be reduced without increasing the diameters of the bottom rolls and whereby the number of spindles in the spinning machine may be increased without experiencing yarn unevenness or breakage.